A standard coupling used between a prime mover constituted by a marine engine and a load constituted by a boat propeller has three stages connected between an input element normally flanged to the flywheel of the engine and an output element that is a hub on a shaft carrying the propeller. The first, second, and third stages are set so that they operate sequentially one after another, first stage first and third stage last, and the second and third stages are connected in parallel to each other and in series with the first stage. The third stage is much softer or easily deformed than the first stage which in turn is softer than the second stage, and formations in the second stage create a lost motion or delay so that the hard second stage only is effective after this lost motion or delay is overcome. Thus as the load is increased to start with the coupling will be relatively soft, with the weak third stage responding. Then the relatively hard second stage will come into action so that at the end the first stage, which is harder than the third stage and softer than the second stage, determines the characteristics of the coupling.
Such a coupling, marketed as a "MerCruiser," is intended to eliminate so-called gear hammering or gear clatter. Such noise is particularly present at low RPM with a diesel engine. With this system, therefore, the input and output elements are indirectly interconnected by the medium-hard elastomeric body of the first stage. When idling or at low engine speed all force is transmitted through this first stage from the engine to the propeller. Only at higher speed does the harder second stage come into action for a more solid connection between the engine and the prop.
To this end the second stage is formed as a pair of coaxial members one of which is formed with an array of axially projecting pins and the other of which is formed with an array of sector-shaped slots into which the pins extend and in which the pins are limitedly angularly movable. Relatively hard elastomeric bodies are provided on the pins or in the slots so that when the pins come to the ends of the slots they rotationally couple the members together. The first stage is connected in parallel to this second stage and is, as mentioned above, substantially softer than the second stage so that when the torque being transmitted through the coupling exceeds a predetermined relatively low level, the second coupling comes into action, and thereafter the response characteristic of the coupling is determined by the series connected first stage. Thus the second stage in effect is phase-shifted relative to the first stage. The characteristic curve of this coupling therefore starts out flat and become quite steep once the second coupling's lost motion is overcome.
The third stage of this known coupling is normally formed by a rubber ring vulcanized onto the hub of the output shaft and tightly surrounded by a metallic ring which is connected to the other two stages that are in parallel. The entire assembly is therefore fairly bulky and, in fact, of substantial axial length. Furthermore the coupling is fairly expensive to manufacture and install.